The inventive concepts relate to a solar cell and a method of fabricating the same. More particularly, the inventive concepts a solar cell including a light absorbing layer and a buffer layer formed at different temperatures from each other and a method of fabricating the same.
A solar cell is a photovoltaic energy conversion system that converts light energy emitted from the sun into electrical energy.
A silicon solar cell uses a P-N junction diode formed in silicon for the photovoltaic energy conversion. However, the silicon used in the silicon solar cell should have a high degree of purity and a low defect density to prevent premature recombination of electrons and holes. Since these technical requirements increase a cost of silicon, the silicon solar cell may have a high fabrication cost. In addition, only photons having energies equal to or greater than a band gap are used to generate a current, and thus, the silicon of the silicon solar cell may be doped with impurities in order to have a lower band gap. However, since electrons excited by blue light or ultraviolet rays have excessive energy due to the low band gap, they may be consumed as heat, so far from generating a current.
Moreover, a P-type layer should be sufficiently thick to increase a probability of capturing photons. However, excited excitons (electron-hole pairs) may be recombined by the thick P-type before they reach a P-N junction, so efficiency of the silicon solar cell may be low.
To solve these problems, a copper-indium-gallium-selenium (CIGS) thin film solar cell is being developed. The CIGS thin film solar cell has efficiency higher than that of the silicon solar cell and relatively high stability. In addition, an initial deterioration phenomenon is not caused in the CIGS thin film solar cell. Thus, various researches are being conducted for commercialization of the CIGS thin film solar cell.
For example, Korean Patent Publication No. 10-2012-0079292 (Korean patent application No. 10-2011-0000509) discloses a hybrid solar cell having a stack structure of the CIGS thin film solar cell and the silicon solar cell (or the CIGS thin film solar cell and a dye-sensitized solar cell) and a method of fabricating the same.
However, indium (In) and gallium (Ga) required for fabricating the CIGS thin film solar cell are expensive, so the fabrication cost of the CIGS thin film solar cell is high. In addition, it is difficult to optimize a co-evaporation process of Cu, In, Ga, and Se, which is used to form the CIGS thin film. Moreover, a cadmium-sulfur (CdS) buffer layer included in the CIGS thin film solar cell is formed by a wet process, so it is impossible to continuously form the CdS buffer layer and another layer formed by a vacuum process (e.g., a sputtering process). This means that productivity of the CIGS thin film solar is low. Furthermore, cadmium (Cd) of the CdS buffer layer is a heavy metal having potent toxicity, and thus, environmental pollution may be caused.
Thus, it is required to develop a solar cell with an eco-friendly characteristic, high-productivity, and a low fabrication cost.